[stdlib] Added NSEnumerator to SequenceType
[stdlib] Added NSEnumerator to SequenceType

Fixes rdar://problem/17925006

Swift SVN r23497

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation